In recent years, there have been growing demands for further reductions in the sulfur content in liquid fuels. In response to these demands, fuel oil producers have already investigated various processes for producing clean fuels. In particular, the adoption of a limit of not more than 10 ppm for the sulfur content of gasoline has lead to fuel oil producers introducing various countermeasures such as improved catalysts and expanded facilities.
Generally, the main base for gasoline is a cracked gasoline produced in a fluid catalytic cracker (FCC). Accordingly, in order to reduce the sulfur content in gasoline, reducing the sulfur content within this cracked gasoline is very important.
The sulfur content within a cracked gasoline is dependent on the sulfur content within the vacuum gas oil that functions as the FCC feedstock, and it is well known that a higher sulfur content within the vacuum gas oil results in a higher sulfur content within the cracked gasoline. Accordingly, in order to produce a clean gasoline having a low sulfur content, the sulfur content must first be removed from the vacuum gas oil that functions as the FCC feedstock.
In a hydrotreating process used for desulfurizing a vacuum gas oil (namely, a FCC pretreatment), a treatment is usually performed in which the vacuum gas oil is subjected to hydrotreating in a fixed bed reactor packed with a hydrotreating catalyst, under a stream of hydrogen gas and under conditions of high temperature and high pressure. A catalyst prepared by supporting an activated metal such as molybdenum or cobalt on a carrier such as alumina is widely used as the hydrotreating catalyst.
It is known that the desulfurization activity during hydrotreating is affected by the type of carrier used, and the type and amount of activated metal used. For example, Non-Patent Document 1 discloses the effects of the carrier (alumina or silica) and the activated metal (molybdenum or a mixture of molybdenum and cobalt). Further Non-Patent Document 2 describes the desulfurization activity of catalysts that use zirconia or titania as the carrier, and nickel or tungsten as the activated metal.